1. Field of the Invention
The present invention relates to an electric charge supplying device used as an charging device for sensitizing a photoconductive member or as an image transfer device for transferring powder images on the photoconductive member to a transfer material such as transfer paper.
2. Description of the Prior Art
In electrophotographic image reproduction systems such as electrophotographic copiers, printers and facsimiles, powder images, developed on a photoconductive member, are transferred to a transfer material such as transfer paper by charging the transfer material with an electric charge supplying device. Also, the photoconductive member is charged by an electrical charging device to sensitize it.
To this end, so far, various charging devices have been used in the image transfer devices. For example, there has been used an electrostatic transfer device comprising a conductive transfer roller of a foamed material that is arranged parallel to a photoconductive drum and brought into contact therewith, and a high-voltage power supply connected to a metal core or shaft of the transfer roller.
In such a device, a sheet of transfer paper is fed to a contacting portion between the photoconductive drum and the transfer roller simultaneously with rotation of the photoconductive drum, and powder images, developed on a photoconductive surface of the drum with a dry developer and composed of charged toners, are transferred to the transfer paper by supplying electric charges with the polarity opposite to that of the charged toners to the transfer paper through the transfer roller serving as a charge supplying member.
However, electrical resistance of the transfer roller and that of the transfer paper are varied approximately two orders of magnitude by changes of environmental conditions such as a temperature and humidity. For example, if the environmental conditions of the transfer device is changed from conditions of normal temperature and normal humidity (hereinafter referred to as "N/N conditions") to conditions of a low temperature and a low humidity (hereinafter referred to as "L/L conditions"), the resistance of the transfer roller is increased several orders of magnitude. In contrast therewith, the resistance of the roller is reduced one or two orders of magnitude under the environmental conditions of a high temperature and a high humidity (hereinafter referred to as "H/H conditions"), compared with that under the N/N conditions.
Accordingly, if the power supply is of a constant-voltage control system designed to keep its output voltage constant, the transfer roller does not provide a sufficient current for the transfer of charged toners under the L/L conditions, resulting in failure in image transfer. Further, under the H/H conditions, the photoconductive drum provides transfer memories during quiescent time of paper feeding, resulting in the printed image with much fogging in the background area thereof.
On the other hand, if the power supply is of a constant-current control system designed to keep its output current applied to the roller constant, an electric current flowing through an area of the roller where the transfer roller is in direct contact with the photoconductive drum increases when the transfer paper fed between the drum and roller is small in size. Thus, an electric current, which flows through an area of the roller where the roller is in contact with the transfer paper, becomes too small to transfer the charged toners from the drum to the transfer paper, resulting in failure in the image transfer.
To solve such problems, it has been proposed in EP-A-0 367 245 to use a power control system (i.e., an active transfer voltage control system, hereinafter referred to as an "ATVC system") which performs the constant-current control of an electric power to be applied to the roller during quiescent time of paper feeding, but performs a constant-voltage control during paper feeding on the basis of the voltage applied to the roller during the constant-current control.
It is, however, essential for the above ATVC system to provide feedback circuits to maintain the output voltage and current constant. Thus, the image transfer device becomes complex in control.